1. Field of the Invention
The present invention relates to a sheet feed device for feeding cut sheets one by one while separating the topmost sheet stacked in a paper cassette.
2. Description of the Related Art
A conventional sheet feed device as disclosed in Japanese Patent Publication No. 2895158 is schematically shown in FIG. 1. The device feeds cut sheets stacked in a paper cassette 206 to a print position and then discharges onto a discharge tray 211. A print head 201 performs printing on a sheet of paper fed to the print position. The print head 201 is mounted on a carriage 202 that is movably supported on a carriage shaft 203 extending in the widthwise direction of the sheet of paper.
The sheet feed device includes a sheet feed roller 207, a sheet feed motor (not shown), a transportation roller 204, a first pinch roller 205a, a second pinch roller 205b, an edge sensor 209, and a discharge roller 210 and its associated pinch roller 210a. The sheet feed roller 207 is disposed above the paper cassette 206. The transportation roller 204 is disposed downstream of the sheet feed roller 207 with respect to the direction in which the sheet of paper is fed. The first and second pinch rollers 205a and 205b are both urged against the transportation roller 204. The edge sensor 209 is disposed in the vicinity of the first pinch roller 205a and also in confrontation with the transportation roller 204. The discharge roller 210 is disposed between the print head 201 and the discharge tray 211.
In operation, the sheet feed motor is driven to rotate the sheet feed roller 207. Then, the sheet feed roller 207 feeds the topmost sheet stacked in the paper cassette 206 toward a nip between the first pinch roller 205a and the transportation roller 204. In cooperation with the first and second pinch rollers 205a and 205b, the transportation roller 204 feeds the sheet of paper fed by the sheet feed roller 207 to the print position. The edge sensor 209 senses the trailing edge of the sheet of paper transported along the transportation roller 204. After the print head 201 performs printing operations on the sheet of paper, the printed sheet is discharged onto the discharge tray 211 in accordance with rotations of the discharge roller 210 and the pinch roller 210a. 
When the sheet of paper has reached the print position 208, the print head 201 performs printing operations while moving back and forth along the carriage shaft 203. Upon completion of one-way movement of the print head 201, judgement is made as to whether or not the trailing edge of the sheet of paper is sensed by the edge sensor 209. The printing operations are repeatedly performed until the trailing edge of the sheet of paper is sensed by the edge sensor 209. When the sheet of paper is printed for a predetermined length after the trailing edge of the sheet of paper is sensed by the edge sensor 209, a sheet feed operation for the subsequent sheet of paper is commenced even if the printing operations are continuing for the preceding sheet of paper.
In the conventional sheet feed device described above, a long interval exists between the two successively fed sheets because the edge sensor 209 is disposed apart from the paper cassette 206. Long sheet-to-sheet interval is disadvantageous in terms of sheet feed efficiency.
In order to shorten the sheet-to-sheet interval, it is conceivable to commence the sheet feed operation before the edge sensor senses the trailing edge of the preceding sheet of paper. In order to accomplish such a sheet feed operation, the timing at which the sheet feed operation is commenced has to be determined based on the estimated position of the trailing edge of the preceding sheet of paper. However, with such sheet feed operations, exact sheet-to-sheet interval is not assured. Moreover, paper jam is liable to occur if a different size of paper is mixed with the sheets of paper stacked in the paper cassette.